This invention relates to a resin transfer molding process in which the use of 2-methyl-1,3-propanediol as the chain extender provides polyurethane elastomers having excellent and unexpected physical properties.
Resin transfer molding is a form of liquid composite molding in which the reaction conditions typically involve longer reaction times, lower temperatures, and lower pressures than the commonly used reaction injection molding ("RIM") technique. In fact, open molds are often suitable for resin transfer molding. Consequently, resin transfer molding is particularly suited to the production of fiber-reinforced plastic articles, for which wetting and impregnation of the reinforcement material is often difficult, and the production of very large articles, for which the high pressure molds used in the RIM technique are impractical or unavailable. See, e.g., E. B. Stark and W. V. Breitigam, "Resin Transfer Molding Materials," and C. F. Johnson, "Resin Transfer Molding," in Engineered Materials Handbook, Vol 1 ("Compositites") (Metals Park, Ohio: ASM International, 1987), pages 168-171 and 565-568, respectively; G. R. Smoluk, Modern Plastics, 66, 57-65 (January, 1989); J. K. Rogers, Plastics Technology, 35, 50-58 (1989); and R. V. Wilder, Modern Plastics, 66, 48-50 (July, 989).
Although the resin transfer molding technique has most commonly been used for polymers other than polyurethanes, the technique can also be used with reactive urethane-based systems. For example, U.S. Pat. Nos. 4,695,509, 4,919,876, and 5,009,821 disclose the preparation of polyurethane composites by resin transfer molding and U.S. Pat. Nos. 4,757,123 and 4,800,058 disclose the preparation of modified rigid polyisocyanurate polymer compositions by resin transfer molding.
An object of the present invention was to find a process for preparing elastomers that gives higher quality product and is more easily controlled than open-mold casting techniques and that, when used to prepare reinforced products, is less likely to disturb or damage the reinforcing materials than reaction injection molding techniques. The present invention provides polyurethane elastomers having unexpectedly high elongation values and high resiliency to compression in combination with overall good physical properties when compared with elastomers extended with 1,4-butanediol using the standard molding techniques.